This invention relates generally to semiconductor lasers, and more particularly, to laser structures for achieving high power outputs, as by incorporating multiple lasing elements into a single device.
By way of background, the basic structure of a p-n junction laser includes an active layer of semiconductor material sandwiched between an n type material and a p type material. A pair of parallel faces of the structure, perpendicular to the plane of the active layer, are cleaved or polished, and the remaining faces are roughened to eliminate lasing in directions other than the desired one. The entire structure is called a Fabry-Perot cavity. When a forward bias voltage is applied across the junction, a current flows. Initially, at low currents, there is spontaneous emission of light from the cavity, in all directions. As the bias is increased, eventually a threshold current is reached at which stimulated emission occurs and a monochromatic and highly directional beam of light is emitted from the junction.
Although many different semiconductor laser geometries have been constructed or proposed, lasers of the double heterostructure type are probably the most widely used. In a double heterostructure (DH) laser, the active layer is sandwiched between two inactive layers that take the form of crystalline solid solutions, such as aluminum gallium arsenide (Al.sub.x Ga.sub.1-x As), where x is the fraction of aluminum arsenide in the material. The DH laser has the advantage of being less temperature dependent and operating at a lower current density than a homostructure laser. Also the DH laser provides a greater difference in refractive index at the boundaries between the active and inactive layers, and therefore confines the light more effectively within the active layer.
Unfortunately, merely increasing the width of a lasing cavity may not result in increased brightness. A wide cavity tends to operate in multiple spatial modes, and the laser light source will then include multiple spots or filaments. This will increase the divergence angle of the resulting beam from the device. For this reason, the brightness, which is the power per unit source area per unit solid angle of the beam, may not be increased at all. Accordingly, there is a need for a more effective technique for providing high power and brightness levels from lasers of this type.
It has been recognized that the use of a curved active layer junction in a semiconductor laser has a desirable current focusing effect at the center of the device. This is discussed generally in a paper by L. Figueroa and S. Wang entitled "Curved junction stablized filament (CJSF) double-heterostructure injection laser," Appl. Phys. Lett. 32, pp. 55-57.
In addition, it has been recognized that mode stabilization of lasers may be achieved by use of a p type substrate and an n type blocking layer for current confinement. See T. Hayakawa et al., "Highly reliable and mode-stabilized (GaAl)As double-heterostructure visible lasers on p-GaAs substrate," Proc. 1981 Intl. Electron Device Meeting, pp. 443-46 (1981).
Various other publications have suggested semiconductor laser structures with curved junctions For example:
R. D. Burnham et al., "Nonplanar large optical cavity GaAs/GaAlAs semiconductor laser," Appl. Phys. Lett. 35, pp. 734-36 (1979);
D. Botez, "CW high-power single-mode operation of constricted double-heterostructure AlGaAs lasers with a large optical cavity, "Appl. Phys. Lett. 36, pp. 190-92 (1980);
D. Botez, "Constricted double-heterostructure AlGaAs diode lasers: structures and electro-optical characteristics," IEEE J. Quantum Electron. QE-17, No. 12, pp. 2290-2309 (1981); and
U.S. Pat. No. 4,215,319 to D. Botez, entitled "Single Filament Laser."
In view of the prior art that has been discussed, there is still a need for further improvement in efficiency, threshold current, and temperature stability of high-power lasers of this general type. The present invention is directed to this end.